Objective Unsteady forces generated by pump-jet rotors are a key factor affecting the vibration and noise levels of underwater vehicles. These forces not only induce structural vibrations through coupling between the propeller, shafting, and hull systems, but also directly determine propulsor noise levels. As pump-jet propulsors represent a promising approach for low-noise underwater propulsion, their rotor unsteady forces exhibit overlapping broadband and line spectra characteristics, with the rotor blade passing frequency (BPF) as the dominant peak. Controlling these unsteady forces is essential for reducing stern structural noise and propeller-induced noise at the source, addressing the challenges posed by complex flow interactions and the difficulty of low-noise optimization in pump-jet systems.

Methods To suppress rotor unsteady forces, this study employs active flow control by integrating actuated oscillating trailing edge flaps into the pump-jet stator. The flaps generate secondary flows that interact with the internal flow, modulating rotor-stator interactions to mitigate unsteady forces. A dedicated pump-jet propulsor featuring oscillating stator trailing edge flaps was designed, comprising a decelerating duct, five stator blades (each equipped with a trailing edge flap covering one-third of the stator chord length), and a four-blade rotor. Open-loop control experiments were conducted in a large circulating water tunnel with a working section of 10.5 m×2.2 m×2 m. A scaled SUBOFF hull model (5 227 mm in length) was employed to replicate the stern wake field, with an unsteady dynamometer installed internally to measure longitudinal unsteady forces. The flaps were actuated by internal actuators following a single-harmonic law to condition the line spectra of the rotor unsteady force under various operating conditions.

Results Under self-propulsion conditions at 6, 8, and 10 kn, the oscillating stator trailing edge flaps demonstrated significant control effects. At the BPF, unsteady forces were reduced by at least 16 dB: 16.4 dB at 6 kn, 20.1 dB at 8 kn, and 17.9 dB at 10 kn, effectively lowering the BPF component to the level of the broadband spectrum. At 6 kn, the shaft frequency and second-order BPF components were suppressed by no less than 7 dB, with shaft frequency reduced by 9.5 dB and second-order BPF by 7.3 dB. The variation trends of the optimal actuation phase and flap swing amplitude were consistent with numerical predictions: the unsteady force amplitude at the controlled frequency varied sinusoidally with phase and quadratically with swing amplitude. Furthermore, system noise measurements indicated that at 8 and 10 kn, overall noise at the BPF was reduced by 4.4 dB and 5.6 dB, respectively, with higher-order harmonics also showing a decline.

Conclusion This study demonstrates that oscillating stator trailing edge flaps effectively suppress the characteristic line spectra of rotor unsteady forces, providing a novel active flow control strategy for pump-jet line spectrum mitigation. The flaps target the BPF, shaft frequency, and second-order BPF components, with control effectiveness maintained across different operating speeds. The observed actuation parameter trends validate the mechanism by which secondary flows reduce inflow inhomogeneity. Overall, the reduction in rotor flow noise and coupled vibration noise surpasses any additional mechanical noise, confirming the feasibility of this approach for low-noise pump-jet propulsion systems.